When graphical information, such as photographs or artwork, is printed in a typical publication, a half-tone printing process is used. Half-tone printing is capable of producing a very high fidelity reproduction of a photograph or artwork if various printing parameters are chosen correctly. However, the half-tone printing process can also introduce some undesirable image distortions when printing parameters are incorrectly chosen. For example, some repetitive patterns in an image (such as a picket fence) may beat against a particular pattern of dots selected for the half-tone process. This causes objectionable repetitive variations in image density which are known as moire patterns or fringes.
In the publication field, the selection of photographs and artwork for a publication has heretofore been done by people who are indirectly involved with the process of half-tone printing. Typically, the selected images are sent to a printer so that a proof sheet can be generated. A proof sheet is made on a press which generates a half-tone reproduction of the selected images. The proof sheets, in half-tone format, are returned to the image selectors. The image selectors then determine whether or not their initially selected images are indeed suitable for the desired publication.
If the images are not suitable, another round of proof sheets may be generated using different images or different half-tone printing parameters. This iterative process can be very costly and time consuming. In order to reduce the costs of generating half-tone proofs, various forms of proof presses have been devised and used. Typically, a proof press is smaller and less costly to operate than a full sized printing press. A proof press is designed so that half-tone printing can be performed in a cost-effective manner for a small number of proof sheets.
However, in spite of the improved cost effectiveness of using a proof press, there remain some substantial problems in getting images selected and ready for inclusion in a publication. For, example, a proof press is a complex and expensive piece of equipment. Consequently, proof presses are typically found only in print shops. Thus a person selecting images is still required to go through the cumbersome process of obtaining proof sheets from a source that is remote or disconnected from his or her office. Additionally, presently available proof presses do not make an exact duplicate of the half-tone reproduction which will be generated by the final press. At best a simulation is made.
A more ideal proof press would be simple to use and inexpensive enough to be placed in the offices of the selectors of images. It would also be capable of producing high fidelity predictions of the appearance of a final output of a half-tone printing press.
Thermal printers are simple to use and have the capability of producing high fidelity images. However, thermal printers have heretofore been impractical for the production of large images that are required as proof sheets. Typically, the desired proof sheets are 22 inches by 17 inches.
In one type of thermal printer, a dye-donor element is placed over a dye-receiving element, and the superposed elements are supported for cooperation with a print head having a plurality of individual heating resistors. When a particular heating resistor is energized, it causes dye from the donor to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the donor. One of the problems in printers of this type is that the thermal time constant of the resistors is quite long. As a result, the printing speed is relatively slow, and the image contrast is limited.
It is known to use lasers instead of the resistors to provide the thermal energy in thermal dye transfer printing. In U.S. Pat. No. 4,804,975, for example, there is shown thermal dye transfer apparatus which comprises an array of diode lasers which can be selectively actuated to direct radiation onto a dye-carrying donor. Radiation from the diode lasers is modulated in accordance with an information signal to form an image on a thermal print medium. The diode laser array extends the full width of the print medium. One problem with this apparatus is that it is too expensive for many applications. The initial cost of such a large array is relatively high, and failure of only one diode laser in the array will result in a discarding of the entire array. A further problem with the patented apparatus is that it is difficult to vary the half-tone dot pattern of the reproduced image.
It is desirable therefore to provide a proof printer which effectively illustrates image artifacts that are a function of interactions of the image a selected pattern of half-tone dots. It is particularly desirable to provide a proof printer that is sufficiently simple to be employed in a typical office setting.